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Phylum Annelida - Q&A Review
1. What are some examples of annelids? Earthworms, leeches and lugworms. Image Diversity: earthworms leeches lugworms 2. Which are the morphological features that differentiate the beings of the phylum Annelida from nematodes and platyhelminthes? Platyhelminthes are worms with flat bodies (flatworms), nematodes are worms with cylindrical but not segmented bodies (roundworms). Annelids are cylindrical worms with segmented bodies (they are metameric). 3. What is the main evolutionary novelty presented by annelids? The main evolutionary novelty presented by the beings of the phylum Annelida is the coelom, the internal body cavity totally covered by mesoderm, a feature also present in arthropods, molluscs, echinoderms and chordates. Platyhelminthes are acoelomate and nematodes are pseudocoelomate (their internal cavity is partially covered by mesoderm). Another important evolutionary novelty of the annelids is the closed circulatory system. Image Diversity: coelom 4. What is the morphological characteristic that evolutionarily approximates the beings of the phylum Annelida to arthropods? The metameric feature, i.e., the body segmentation in metameres, approximates annelids to arthropods since these animals are segmented beings too. (Bristles present in oligochaete and polychaete annelids are also covered with chitin, the same substance of the arthropod exoskeleton.) 5. How does digestion in beings of the phylum Annelida work and which type of digestive system do they have? Digestion in beings of the phylum Annelida is extracellular. These animals have a complete digestive system, with mouth and anus. Image Diversity: earthworm digestive system 6. Which are the characteristics and organs of the digestive system of earthworms related to the type of diet of these animals? Earthworms eat decomposing organic material and small organisms ingested together with soil particles. The digestive tubes of earthworms have special structures, like a muscular wall and a gizzard, that triturate the food and scratch it against the ingested soil particles. Since annelid digestion is exclusively extracellular earthworms also present in the posterior part of their digestive system structures like the cecum and the typhlosole that have the function of increasing the absorption surface of the intestine. 7. The vascular lesions caused by leeches upon the blood vessels of their host cause blood naturally to coagulate. How does the leech solve this problem since it could be expected that the ingested blood would coagulate inside its body? Ingested blood does not coagulate inside the leech (Hirudo medicinalis) because in its saliva there is a potent anticoagulant substance, a protein called hirudin. In the past leeches were largely used as medical treatment. Nowadays hirudotherapy is being used in patients with extensive and chronic inflammation of the skin, in prevention against tissue necrosis after some surgeries and in several others fields of Medicine. 8. How is the respiratory system of beings of the phylum Annelida characterized? Respiration in annelids can be cutaneous or branchial. Cutaneous respiration occurs due to the rich vascularity under the epidermis. The gills, present in aquatic annelids, are located in the parapodia (false claws) that have an extensive capillary net. Image Diversity: annelid respiratory system 9. What is meant when it is said that beings of the phylum Annelida are vascular beings? From which other phyla of the animal kingdom does this feature differentiate them? The classification of these beings as vascular beings means that they have a circulatory system, with vessels that distribute substances throughout the body. Poriferans, cnidarians and flatworms do not have a circulatory system. In nematodes there is circulation of gases and nutrients through the pseudocoelom fluid. 10. How are the circulatory systems of animals classified? A circulatory system is classified as open or closed. In open circulatory systems blood gets out of vessels and flows also to large cavities that perfuse the tissues to be irrigated. In closed circulatory systems blood circulates only within blood vessels and through the heart. Image Diversity: closed circulatory system open circulatory system 11. What is the type of circulatory system present in annelids? In beings of the phylum Annelida the circulatory system is closed, i.e., blood circulation takes place only within specialized vessels. 12. Is there a respiratory pigment in the annelid blood? The blood in beings of the phylum Annelida contains the respiratory pigment hemoglobin (the same found in chordates) and other pigments too. 13. How can the presence, localization and function of muscular tissue in beings of the phylum Annelida be explained? In these beings there are a longitudinal muscular layer under the epidermis and, internally juxtaposed and perpendicular to it, another circular (radial to the axis) muscular layer. The circular muscle layer has the function of elongating the body while the longitudinal shortens it. By alternating actions both promote movement. 14. How can the excretory system of annelids be described? In each segment (metamere) of the being a pair of complete excretory structures called metanephridium exists. The metanephridium has an extremity, the nephrostoma, which collects residuals from the coelom, filtering them and causing reabsorption along its extension (similar to human nephron tubules). The material to be excreted goes out through a pore, the nephridiopore, which opens in the body surface. Image Diversity: earthworm excretory system 15. How is the nervous system characterized in beings of the phylum Annelida? How can one compare cephalization in annelids to cephalization in nematodes and platyhelminthes? Annelids have a nervous system made of two ventral chords and one relatively big nervous cell concentration in its anterior portion resembling a primitive brain. Nematodes have an anterior neural ring connected to two neural chords, a ventral and a dorsal one, while in planarias (platyhelminthes) there are only two small anterior “cerebral” ganglia from which neural chords split. Cephalization in annelids thus is more outstanding than in nematodes or in flatworms. Image Diversity: earthworm nervous system 16. What is the clitellum of earthtworms and where it is located? The clitellum is a special region of the annelid constituted by rings (metameres) with reproductive function. It can be found in the anterior portion of the animal and it is characterized by a lighter color in comparison to the normal color of the other segments. 17. Concerning the occurrence of separated sexes how are the beings of the phylum Annelida classified? These beings may be dioecious (the majority of polychaetes) or hermaphrodite monoecious (oligochaetes and hirudineans). 18. Is the embryonic development in earthworms direct or indirect? In earthworms there is no larval stage, so the embryonic development is direct. 19. What is the name of the larval stage of polychaetes? Among the annelid classes only polychaetes present a larval stage. Their larva is called trocophore. 20. What is the ecological role of earthworms? Earthworms have an important ecological role as they eat decomposing organic material. They also dig tunnels in the subsoil allowing the entrance of gases and nutrients that are useful for plant roots and other living beings. So they act as decomposers and as fertilizers too. 21. Into which classes is the phylum Annelida divided? The phylum is divided into three classes: oligochaetes (for example, earthworms), hirudineans (e.g., leeches) and polychaetes (these are mostly marine aquatic with parapodia, like nereis). Image Diversity: oligochaetes polychaetes hirudineans 22. Annelid Identity card. How are they characterized according to examples of representing beings, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction? Examples of representing beings: earthworms, leeches, lugworms. Basic morphology: cylindrical body, segmented (metameric). Type of symmetry: bilateral. Germ layers and coelom: triploblastics, coelomates. Digestive system: complete. Respiratory system: cutaneous or branchial. Circulatory system: closed, with hemoglobin. Excretory system: a pair of metanephridia in each metamere. Nervous system: neural chords, a pair of ganglia per metamere, anterior concentration of neurons (primitive brain). Types of reproduction: sexual, with dioecious and monoecious beings. |